1. Field of the Invention
The invention pertains to the art of automatic flue dampers for furnace exhaust gas flues, and in particular to the type in which the damper plate is mechanically actuated in response to changes in the flue gas temperatures.
2. Description of the Prior Art
Automatic flue dampers are arranged to be actuated in a number of different ways. These ways include electric motors, solenoids, pneumatic or hydraulic actuation, as well as using the thermal expansion or vaporization of various solids or liquids to position the flue damper. Additionally it has been proposed to use bimetal actuated arrangements which include at least one arrangement in which four bimetal pieces arranged in quadrants in the flue obstruct the flue gas and then bend out of the gas path when they are heated by the gas. Another bimetal actuated arrangement shown in U.S. Pat. No. 3,366,333 uses a bimetal spiral which operates a flue damper between open and closed positions.
The non-bimetal type arrangements require a fair degree of mechanical complexity, particularly in that the arrangements require, to be fail-safe, that the dampers open even if hot gases reach them unaccompanied by an electrical command to open. The prior art bimetal design in which quadrants of bimetal are used as somewhat complex in that each of the quadrants of the flue must have its own bimetal flapper and this requires a significant amount of bimetal material to cover the flue area. Furthermore this quadrant type design does not lend itself to furnishing an external indication of the position of the bimetals in the flue and it is also not convenient from outside the flue pipe to force or hold open this style of damper.
As to the noted patented bimetal actuated arrangement, this arrangement is considered deficient with respect to mine in that it requires the use of a fixed shaft which precludes the external indication of the damper position, and the bimetal in a spiral conformation does not provide as good thermal interaction between the bimetal and flue gas since the outer spiral parts tend to block the quick heat transfer to the inner spiral material. The arrangement also requires the provision of a frame within the flue with two integral offset semi-circular portions to serve as stops for the damper.
The use of a helical bimetal, is distinguished from a spiral bimetal, and which is responsive to changes in temperature is shown in U.S. Pat. No. 2,284,674. This arrangement is shown in conjunction with a hot air furnace in which the damper is in the supply duct from the furnace to the rooms, as distinct from in the furnace flue which receives the exhaust gases. The arrangement there shown is also considered to have mechanical and assembly complexities in comparison to the relatively simple and inexpensive arrangement of my invention.